In general, various operational troubles occur with the aging of a coke oven. Among such operational troubles, “sticker”, which can hamper the discharge of manufactured coke from the oven, is an extremely serious operational trouble. When “sticker” occurs, a manufacturing schedule of coke is forced to be changed and a manufacturing amount of coke is thus decreased. In addition, the occurrence of “sticker” causes damage to an oven body, resulting in a short lifetime of the coke oven.
The general mechanism of “sticker” is as follows. In operation of general chamber-type coke ovens, blended coal charged into a carbonization chamber is sequentially carbonized from the oven wall side with heat coming from a combustion chamber adjacent to the carbonization chamber and a cake of coke is generated. In normal operation, the cake of coke itself shrinks by carbonization, and spaces (hereinafter, referred to as clearance) are formed between oven walls and the outer surfaces of the cake of coke. The formation of the clearance enables the cake of coke to be discharged (extruded) to the outside of the oven easily.
When clearance having a sufficient size is not formed due to insufficient shrinkage of the cake of coke, “sticker” occurs because of increased frictional resistance between the oven walls and the outer surfaces of the cake of coke in discharging the cake of coke. Furthermore, when the surfaces of the oven walls have large irregularities, “sticker” occurs because of increased frictional resistance between the oven walls and the outer surfaces of the cake of coke in the same manner.
The irregularities on the surfaces of the oven walls increase under influence of abrasion and drop of bricks of the oven walls, growth of carbon attached to the oven walls, and the like with advancement of deterioration of an old coke oven. That is, the occurrence frequency of “sticker” inevitably increases with the aging of the coke oven. In consideration of these circumstances, various measures have been taken to reduce the occurrence frequency of “sticker” in operation of the old coke oven.
As a measure to reduce the occurrence frequency of “sticker”, a wet coal operation, in which the coke oven is operated without actively decreasing the moisture content of blended coal from the moisture content (approximately 8 to 14% by mass although it fluctuates depending on the season and the weather) of the blended coal when it has been piled up in a yard, has been widely employed as the simplest and the most effective means. An increase in the moisture content of the blended coal lowers the charging bulk density of the blended coal and enlarges the clearance, for example, and thus the frictional resistance between the oven walls and the outer surfaces of the cake of coke in discharge is decreased, thereby reducing the occurrence frequency of “sticker”.
To be specific, Patent Literature 1 discloses a technique of carbonizing blended coal in a coke oven after the moisture content of the blended coal is adjusted using a coal moisture-control facility. The technique involves calculation of a target moisture content of the blended coal that is necessary for ensuring a desired clearance based on a relation between the moisture content of the blended coal and the clearance previously measured. With this technique, heat input into the coal moisture-control facility is controlled such that the total moisture content of the blended coal at the output side of the coal moisture-control facility will become the target moisture content, thereby reducing the occurrence frequency of “sticker”.
Furthermore, Patent Literature 2 discloses a technique by which water is locally added to coal in a coal tower for supplying the coal to a coal charging car that charges the coal into a carbonization chamber and the coal to which water is added is charged into the carbonization chamber through the coal charging car. With this technique, the coal having an increased moisture content relative to other coal is unevenly distributed into a part of the carbonization chamber, and thus the shrinkage rate of the coke on coal portions having the increased moisture content is increased and the clearance is enlarged, thereby reducing the occurrence frequency of “sticker”.
As described above, the increase in the moisture content of the blended coal is effective in reducing the occurrence frequency of “sticker”. Note that many coke ovens introduce a process of decreasing the moisture content of the blended coal using a moisture control facility or preheating facility into a pretreatment process of the blended coal in order to improve coke strength, for example. The reduction in the occurrence frequency of “sticker” is, however, the most important matter for operation with the old coke oven.
For this reason, the moisture content of the blended coal cannot be decreased even when improvement in the coke strength is required, and the moisture content of the blended coal tends to be increased. Gas permeability and liquid permeability need to be ensured in a blast furnace for stably operating the blast furnace using coke manufactured in a coke oven, and coke that has excellent strength, in particular, drum strength, which is measured by the drum strength test method according to JIS K 2151, is essential. In view of these circumstances, techniques of improving the strength of coke have been developed.
The techniques of improving the coke strength are largely classified into a pretreatment technique, a blending technique, and a carbonizing technique. Among them, the pretreatment technique enables a facility to be designed so as not to impose restriction on productivity of a coke oven without increasing the cost of blended coal, and is thus considered to be particularly important. The pretreatment technique is largely classified, based on the way of approach to the coke strength, into two techniques: (1) a technique of improving the charging bulk density of the blended coal (hereinafter, referred to as technique (1)) and (2) a technique of homogenizing the blended coal (hereinafter, referred to as technique (2)).
An object of the technique (1) is to reduce spaces between coal particles when the blended coal is charged into the coke oven in order to reduce the number of pore defects, which can influence the coke strength. Methods of the technique (1) include: a method by which the blended coal is mechanically consolidated and is charged into the coke oven, examples thereof include a method by which some coal briquette is charged and a stamping method; and a method of improving the charging bulk density by decreasing the moisture content of the blended coal and making attachment force between the coal particles weak, examples thereof include a coal moisture control method, preheated coal charging, the dry-cleaned and agglomerated precompaction system (DAPS), and the super coke oven for productivity and environmental enhancement toward the 21st century (SCOPE-21) (see Non-Patent Literature 1).
By contrast, an object of the technique (2) is to raise the strength of a portion of the coke that has the lowest strength. The coal is originally composed of textures having different thermal and mechanical characteristics and is extremely non-homogeneous, and the texture of coke manufactured from the non-homogeneous coal is also non-homogeneous. The strength of a brittle material such as coke is generally described based on a weakest link model, and is determined by the strength of the portion having the lowest strength in the material. Accordingly, homogenization of the texture of the coke can average the strength in the coke, thereby raising the strength of the portion of the coke that has the lowest strength and improving the strength of the entire coke.
Methods of the technique (2) include a method by which the particle size of coal is adjusted (see Non-Patent Literature 1). The method by which the particle size of the coal is adjusted basically aims at homogenization of the texture of the coke by finely grinding the coal. Also known is a method of homogenizing the texture of coke by processing coal with a coal mixer such as a drum mixer and enhancing the mixing degree of the coal (see Non-Patent Literature 2). Meanwhile, conventional studies have revealed that blended coal to be used in a coke manufacturing process is sufficiently mixed by connection of belt conveyors during conveyance, for example, without passing through a coal mixer (see Non-Patent Literature 2). For this reason, many coke plants have taken measures to homogenize the texture of coke without using coal mixers.